This invention relates to optical amplifier manufacture, and is particularly concerned with the spectral gain characteristics of such amplifiers.
Any given optical amplifier has a finite spectral band over which the amplifier affords optical amplification. Clearly the amplification that it does afford will fade away in magnitude toward the edges of that band. However it has been found that, in the absence of any spectral filtering, the spectral gain characteristic of such an amplifier, for instance an amplifier whose gain medium is constituted by one or more lengths of optically pumped rare earth doped (typically erbium doped) optical fibre, exhibits a marked departure from flatness, not only near the band edges, but also over the intervening spectral range, and also over smaller portions of that intervening range. For many practical applications this lack of flatness is a disadvantage. For instance, if a wavelength division multiplexed WDM signal is to be amplified, it is generally desirable for the amplifier to exhibit substantial flatness over the whole spectral range compassed by the WDM signal. Within at least some portion of the central region of the full amplification band, any specific non-flat spectral gain characteristic can in principle be compensated by the use of a corresponding gain-flattening filter, such as that made by the technique described in U.S. Pat. No. 5,708,740. The spectral profile of any given example of such a filter is a fixed (static) profile, and so is matched only to one specific spectral profile of gain characteristic. The usefulness of such filters for gain-flattening is therefore limited by the fact that the spectral gain characteristic of the amplifier typically varies significantly with drive conditions, a phenomenon known as gain tilt.
In particular the effect of increasing the drive is to increase the gain at shorter wavelengths proportionately more than at the longer wavelengths, i.e. the gain characteristic, when gain is plotted as a function of wavelength, is tilted clockwise. A similar clockwise tilt can alternatively be obtained, not by increasing the drive, but maintaining it in such a way as to provide the same value of gain at some reference wavelength, and instead arranging for the gain to take place over a shorter length of gain medium. It is therefore possible to arrange compensate the clockwise tilt resulting from an increase in drive with a counter-clockwise tilt resulting from the use of a longer length of gain medium.
One of the factors determining the spectral gain characteristic of an optical amplifier whose gain medium is provided by one or more concatenated lengths of optically pumped rare-earth doped optical fibre is the particular `recipe` (dopant choice, doping level, refractive index profile, core diameter etc) used in the construction of that gain fibre. For a given fibre length, the shape of the gain characteristic is not fully determined by the external gain of the amplifier (i.e. the gain the amplifier shows to the external world between its input and output ports, it is instead determined by the internal gain of the amplifier. The difference between the internal and external gain values of an amplifier is equal to the aggregate loss of the passive components of the amplifier that are in series with the or each region of gain fibre in the optical path optically coupling the input port of the amplifier to it output port.
Consider now the case of an amplifier having, at some particular reference wavelength, .lambda..sub.ref, an external gain of xdB, aggregate loss of its passive components being ydB, and its external gain being zdb (where x+y=z). Not only is the internal gain value of this amplifier determined, but also its gain medium recipe and length. Therefore the spectral gain characteristic of the gain medium of the amplifier is determined, and therefore it is possible, at least in principle, to construct a gain flattening filter for use in series with the amplifier, either before or after it, that will provide two series combination with optimum spectral flattening (over a predetermined spectral range within the gain medium) when the amplifier is driven in such a way as to provide it with an external gain of xdB.
Now suppose that gain flattening is wanted for a different amplifier required to be driven so as to provide an external gain value of (x+.DELTA.x)dB at .lambda..sub.ref. Clearly the same gain medium recipe and length can be used, provided that the aggregate loss of the passive components can be reduced to (y-.DELTA.x)dB. An alternative approach would be to employ a drive providing the amplifier with an internal gain of (z+.DELTA.z)dB at .lambda..sub.ref, and to compensate for the gain tilt produced by the gain increment by the use of a complementary gain tilt produced by the use of an incrementally lengthened gain medium.
In principle therefore the same design of gain flattening filter can be used for producing optimised gain flattening for amplifiers with different specific value of external gain, choosing in each instance the appropriate length of gain medium having regard to the external gain that that amplifier is required to provide, and also to the aggregate loss value of its passive components.
Only for simplicity of exposition has the foregoing analysis treated the gain flattening filter as being external to the amplifier. However since the gain flattening filter will normally present finite loss, .DELTA.ydB at .lambda..sub.ref, the external gain of the series combination of amplifier and gain flattening filter will be .DELTA.ydB less than that of the amplifier alone. Normally it is the external gain of the series combination that is significant to the system designer, and so it will generally be more appropriate to treat the gain flattening filter as being an internal constituent part of the amplifier. Under these circumstances the loss .DELTA.ydB presented at .lambda..sub.ref by the gain flattening filter is incorporated as part of the aggregate loss of the passive components of the amplifier.
For a given gain medium recipe it is possible, not only to determine the spectral gain characteristic for a specific value of internal gain and gain medium length, but it is also possible to determine how the spectral gain characteristic changes both as a function of internal gain magnitude and as a function of gain medium length. Therefore, in respect of the construction of an amplifier that employs that recipe of gain medium, and that incorporates within it a specific design of gain flattening filter, it is in principle possible to select the requisite length of gain medium to achieve optimised gain flattening for a specific value of external gain at .lambda..sub.ref once the value of the aggregate loss of the passive components of that amplifier at .lambda..sub.ref is known. As estimate of this aggregate loss can be arrived at by assembling all the components of the amplifier except for the length of lengths of amplifying medium (optical fibre) of that amplifier. The place of the or each such length of amplifying fibre is taken by a temporary fusion splice. The actual aggregate loss of these components can then be measured. To this value is next added the expected loss increment involved in replacing the or each temporary fusion splice with two permanent fusion splices required for insertion of the previously omitted length of amplifying fibre. Unfortunately there can be an unacceptably large discrepancy between the computed value of the aggregate loss and the value actually resulting from the splicing-in of the amplifying fibres.
It would therefore be beneficial to be able to include, within an amplifier that includes a gain flattening filter, a neutral density filter whose attenuation could be trimmed after completion of the construction of that amplifier. Such a neutral density filter could then be used to optimise the gain flattening for a pre-set value of amplifier external gain. (For the purposes of this specification the term `neutral density filter` is employed in the context of an optical amplifier, to mean a filter having a spectral attenuation characteristic that is substantially flat over the amplification waveband of that amplifier.) Such a neutral density filter could additionally be used to compensate for the effects of small changes in fibre recipe that are typically found to occur along the length of a reel of amplifier whose composition is nominally the same throughout, and to also compensate for small changes between amplifying fibres derived from different reels. In this context, a reel of fibre may comprise several kilometers of fibres all drawn from a single fibre preform of nominally uniform composition.